Processor Organization and Architecture Module III Processor Organization and Architecture

Nano-programming In this method, microinstructions does not generate control signals directly but uses a second control memory called Nano Control Memory (NCM) There are two levels of control memory: higher level Micro Control Memory & lower level NCM that stores nano-instructions

Nano-programming Suppose there are n = 2048 micro-ops each of 41 bits wide, The memory required for single control ROM is 2048 x 41 =83,968 bits

Nano-programming Suppose there are 100 unique control words in ROM. These 100 words can be uniquely saved in nano control ROM saving space as shown:

Nano-programming Advantage: Disadvantage: Reduces total size of required control memory Greater design flexibility Disadvantage: Less speed due to two level memory

Classification based on instruction sets Complex Instruction Set Computers Reduced Instruction Set Computers

Complex Instruction Set Computers CISC processor mostly uses a unified cache for both data and instructions and shares the same path They mostly use microprogrammed CU and hence control memory is required and it slows down the execution

CISC Architecture

CISC Characteristics It tries to simplify the compilation and improve the overall computer performance. CISC architecture attempt to provide a single machine instruction for every statement in a high-level language. It incorporates variable-length instruction formats. Packing variable instruction formats in memory requires special decoding circuits.

CISC Characteristics Instructions manipulate operands in memory Uses a variety of addressing modes—from 5 to 20 Uses a large number of instructions—from 100 to 250 Includes specialized instructions which are used infrequently Examples : Motorola 68000 family, Intel x86 CPUs-Pentium

Reduced Instruction Set Computers It uses separate instruction and data caches and their access paths are different It uses hardwired control unit and hence faster execution.

RISC Architecture

RISC Characteristics Uses a small set of instructions with register-to-register operations and simple load and store operations for memory. Uses only a few addressing modes Follows a simple instruction format instruction length is fixed and aligned on word boundaries. Instructions are easy to decode. Hardwired control makes faster execution

RISC Characteristics Execute one instruction per clock cycle Includes pipelining Load or store may require two clock cycles. Uses a large number of registers overlapped register windows speed-up procedure call and return. Thus register-to-memory operations is minimized Examples : SPARC, Alpha,...

RISC vs CISC RISC CISC Contains simple instructions of one clock cycle Very few instructions refer to memory Hardwired Control Unit Fixed Format Instructions Contains complex instructions taking more than one clock cycle Most of the instructions may refer memory Micro-programmed Control Unit Variable Format Instructions

RISC vs CISC RISC CISC Few Instructions Few Addressing Modes Many Registers Complexity is in compiler Faster clock rates Reduces the cycles per instruction at the cost of the number of instructions per program. Many Instructions Many Addressing Modes Few Registers Complexity is in microprogram Slower clock rates Minimize the number of instructions per program sacrificing the number of cycles per instruction.